Method of group iii-v semiconductor crystal growth using getter dried boric oxide encapsulant

ABSTRACT

The impurity content of boric oxide for use as the encapsulant in the growth of crystals of Group III-V semiconductors by the liquid encapsulation technique is reduced by a high-temperature chemical treatment. The boric oxide is maintained for a time together with a getter substance under vacuum at a temperature at which both are molten. When the growth of crystals of GaAs and GaP is contemplated, some suitable getter substances are boronpalladium alloy, boron-platinum alloy and the pure metals or alloy combination of gallium, aluminum and silicon.

United States Patent Weiner 1 Mar. 7, 1972 [54] METHOD OF GROUP III-V SEMICONDUCTOR CRYSTAL GROWTH USING GETTER DRIED BORIC OXIDE ENCAPSULANT [72] Inventor: Martin Eric Weiner, Bridgewater Township, Somerset County, NJ.

[73] Assignee: Bell Telephone Laboratories, Incorporated,

Murray Hill, Berkeley Heights, NJ.

[22] Filed: May 11, 1970 211 App]. No.2 36,367

[52] US. Cl. ..23/204 R, 23/295 [51] Int. Cl. ..C0lb 27/00, BOld 9/00 [58] Field of Search ..23/204 R, 295; l48/l.6:

[56] References Cited OTHER PUBLICATIONS Mullin et 21].: Liquid Encapsulation Techniques, .1. Phys.

Chem. Solids, Vol. 26 (1965), pp. 782- 784 PochzGlastechnische Berichte, Vol. 37 (1964), pp. 533- 535 Goetzherger et aL: Metal Precipitates in Silicon PN Junctions J. ofApplied Physics Vol. 31 (1960), PP- 1821- 1824 Primary Examiner-Oscar R. Vertiz Assistant ExaminerHoke S. Miller AttorneyR. .I. Guenther and Edwin B. Cave [5 7] ABSTRACT The impurity content of boric oxide for use as the encapsuiant in the growth of crystals of Group lll-V semiconductors by the liquid encapsulation technique is reduced by a high-temperature chemical treatment. The boric oxide is maintained for a time together with a getter substance under vacuum at a temperature at which both are molten. When the growth of crystals of GaAs and 6a? is contemplated, some suitable getter substances are boron-palladium alloy, boron-platinum alloy and the pure metals or alloy combination of gallium, aluminum and silicon.

5 Claims, 2 Drawing Figures VACUUM SYSTEM PATENTEUMAR 1 m2 3.647, 389

l9 VACUUM SYSTEM FIG. 2

VACUUM SYSTEM lNl/ENTOR M. E. WE/NER V METHOD OF GROUP III-V SEMICONDUCTOR CRYSTAL GROWTH USING GETTER DRIED BORIC OXIDE ENCAPSULANT BACKGROUND OF THE INVENTION 1. Field of the Invention The invention lies in the field of semiconductor crystal growth.

2. Description of the Prior Art A liquid encapsulation technique has been developed for use in the growth of compound semiconductor crystals. The particular problem towhich this technique is directed is the preferential loss of one constituent due to the difference in volatility of the various constituents and dopants of the compound semiconductor. For instance, liquid gallium phosphide will rapidly lose phosphorus unless suitable precautions are taken. Such a preferential loss of one constituent can be greatly reduced if an inert liquid is floated on top of the semiconductor liquid and an overpressure of an inert gas is maintained in the space above the liquid greater than the vapor pressure of the more volatile constituent 50 atmospheres for the phosphorous referred to above). For the group III-V compound semiconductors, boric oxide (B has proven to be a satisfactory encapsulant. It does not react with these compounds and is available in a high purity form. The liquid boric oxide wets both the grown semiconducting crystal and most common growth vessel materials so that it forms a somewhat protective coating over the crystal and, in some measure, protects the melt from contamination by the vessel.

Boric oxide however is highly hygroscopic and even most high-purity materials contain sufficient quantities of water to cause the boric oxide to froth and spatter when melted. A treatment has been developed to decrease the amount of water present in boric oxide to be used in crystal growth. The material is subjected to a vacuum baking treatment in which the boric oxide is melted and maintained under vacuum at a temperature in excess of I,0O0 C. for times ranging from many hours to several days. Boric oxide produced by this rather extreme process has proven, generally, quite satisfactory for crystal growth and is widely used.

Although greatly improving the crystal growth situation, the liquid encapsulation technique as described above, does not solve all of the crystal growth problems. During the melt down of, for instance, gallium arsenide under vacuum-baked boric oxide considerable bubbling is still observed and reaction products, which contaminate the crystal growth system, are produced. For instance, in the case of gallium arsenide, arsenic is deposited over the inside of the vacuum chamber at times obscuring the viewing ports. Considerable effort has gone into an attempt to locate the source of these problems and effect improvements.

SUMMARY OF THE INVENTION It has been found that the electric properties of Group III-V semiconductor crystals grown using the liquid encapsulation technique can, be improved and the contamination of the growing apparatus can be reduced if the boric oxide is subjected to a gettering procedure. This procedure involves maintaining the boric oxide together with a getter substance at a temperature of greater than the melting point of both the boric oxide and the getter substance for times ranging from of .the order of several hours to several days and subjecting this melt to reduced pressure. It is believed that the improvement results at least in part from the getter substance chemically reacting with any water remaining in the boric oxide after the vacuum-baking procedure. Volatile reaction products are substantially removed by the vacuum system and many solid and liquid reaction products and other impurities are dissolved in the getter substance. This gettering step can also be incorporated in the vacuum baking procedure. For use in the growth of gallium phosphide, gallium arsenide, and mixed crystals thereof suitable getter substances are boron-palladium alloy, boron-platinum alloy, gallium aluminum, silicon or alloys of these latter three metals.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a plan view in section of a getter drying apparatus; and

FIG. 2 is a plan view in section of a Czochralski crystal growth apparatus using the liquid encapsulation technique.

DESCRIPTION OF THE INVENTION A number of conditions must generally be met by materials to be used as getter substances in the disclosed process for the chemical treatment of boric oxide. The proposed getter substance must be capable of reducing H (the high-temperature hydrated form of 8 0 at the drying temperature. Its oxide must be soluble in boric oxide and must be either volatile (hence removable by vacuum baking) or should not possess elements which would deleteriously contaminate the semiconductor melt. The getter substance should be a liquid at the temperature at which drying is carried out so as to be capable of dissolving nonvolatile impurities. It is also advantageous that the getter substances be more dense than boric oxide since the simplest way to separate the getter substance from the boric oxide after drying is to melt the boric oxide and pour it oft" of the getter substance. Although this is not a strict requirement.

Considering these requirements as applied to the growth of gallium phosphide, gallium arsenide and mixed crystals thereof, a number of suitable getter substances have been found. Alloys of boron-platinum and boron-palladium possess a steep eutectic, melting in the neighborhood of 830 to 850 C. Alloys of these substances in the neighborhood of eutectic composition are suitable as getter substances. The oxide produced by the reduction of water is boric oxide and thus does not introduce any new species into the boric oxide encapsulant. The platinum or palladium are nonreacting in this system.

Gallium, as a getter substance, introduces a new species into the boron oxide in that some gallium oxide remains after the drying treatment. However, gallium is not a contaminant in the growth of gallium phosphide and gallium arsenide and the oxygen levels are reduced below the levels produced by the presence of H 0. Oxygen is a deep trap in gallium arsenide and-an important dopant in gallium phosphide which must be carefully controlled.

Aluminum, appearing just above gallium in the periodic table of the elements, is not an electrically active contaminant. It is thus an acceptable material. Also aluminum oxide is more stable than gallium oxide or boric oxide and, therefore, leads to even less oxygen contamination of the semiconductor melt.

Crystals of Ga? and GaAs are often grown from quartz crucibles. This leads to some silicon contamination. Even though the use of silicon as a getter substance will lead to the presence of silicon dioxide in the boric oxide, this will not be the controlling factor in the silicon contamination of the crystal. Thus, silicon is an acceptable getter substance for boric oxide intended for quartz crucible growth.

The gettering process should take place at a temperature above which both the boric oxide and the getter substance are molten in order to promote the chemical drying process and the solution of other impurities in the getter substance. The melting point of boric oxide is approximately 460 C. although because of its viscosity a preferred lower temperature for the gettering process is 600 C. The temperature of the gettering process should be kept below l,500 C. because of excessive boric oxide evaporation above this temperature. In order to remove the volatile reaction products. the drying process must take place under reduced pressure. The time occupied by this process varies from about an hour to several days. 0ne reason for this large time variation is the variation of the viscosity of boric oxide with temperature. At the lowest temperatures, the molten boric oxide is extremely viscous so that the pressure above the melt must be reduced slowly in order to avoid excessive frothing produced by the volatile reaction products. At the highest temperatures the boric oxide is much less viscous and the pressure can be more rapidly reduced (within one hour or more preferably within 5 hours) to as low as the l micron level without causing excessive loss of boric oxide due to frothing. Experiments which have been performed during the investigation of this process have generally taken place in the neighborhood of 900 to 1,100 C. Enough of the getter material must be used in order to react with all of the water in the boric oxide. However, much more than this quantity is generally employed since the speed of the reaction depends in part on the surface area of the molten getter substance.

FIG. 1 shows the elements of an exemplary getter drying apparatus. The boric oxide 11 and the getter substance 12 are contained within a crucible 13 which is situated in a chamber 14, connected through an orifice 15 to a vacuum system 19. The required treatment temperature is maintained by heating coils 16.

In one exemplary drying process 80 grams of boric oxide were melted together with grams of gallium and 0.2 grams of aluminum in an aluminum oxide crucible. These were maintained at 1,000 C. and held at /2 atmosphere pressure overnight by a vacuum system 19 including suitable pumping and "pressure regulating apparatus. Over the next 48 hours, the

pressure was successively reduced until a pressure of microns was reached. The melt was maintained at this pressure for several hours. in another exemplary drying process 80 grams of boric oxide was melted together with one gramof aluminum and maintained at l000 C. with a similar timepressure schedule. At any one temperature, the time schedule can be minimized by diligent and continuous observation of the melt in order to maintain the pressure reduction rate just low enough to prevent the froth 17 from running over the side of the crucible 13. At l,000 C., the time can be reduced to of the order of 12 hours by such diligence.

Fig. 2 shows some of the elements of a Czochralski crystal growing apparatus making use of the liquid encapsulation technique. This is one of the many crystal growing techniques known in the art which can make use of liquid encapsulation where this proves advantageous. Here the semiconductor melt 27 and the boron oxide encapsulant 21 are contained within a crucible 23 and maintained at the proper growing temperature by means of the heating element 26. During the growth process a rotating rod 29 with a seed crystal attached to its lowest end is lowered to make contact with the free surface 22 of the semiconductor melt 27 and initiate crystalization. It is then slowly raised pulling" crystal 28 from the melt through the encapsulant 21 which usually provides a coating over the crystal 28. The pressure-regulating system 31 maintains the required overpressure of inert gas.

It appears that a vacuum baking process however extreme cannot lower the water content of ordinary boric oxide to less than 0.02 weight percent. However, boric oxide can be produced by the disclosed process with less than 0.0] weight percent water. Water contents a factor 10 less than this and lower have been attained in boric oxide produced by a gettering process such as produced by an apparatus shown in FIG. 1.

Crystal growth'by an apparatus such as shown in FIG. 2 is greatly facilitated since the bubbling of the liquid encapsulant 21 is greatly reduced and the deposition of reaction products on the walls of the vacuum chamber 24 and the consequent obscuration of the viewing port 30 are greatly reduced. The effects of the disclosed techniques can also be observed in the electrical properties of the pulled crystal 28. Exemplary crystals produced by this technique have shown such electrical improvements as increase of carrier mobility which can be traced to a lowering of the impurity content of the semiconductor material 28.

What is claimed is:

1. Method for the formation of a crystalline body of a Group Ill-V compound semiconducting material from a molten body of said semiconducting material corn rising holding said molten body in a container such that sat molten body has a free surface the free surface of which said molten body is covered by a molten layer of boric oxide characterized in that the said boric oxide has, prior to said formation, been treated for the removal of impurities by maintaining said boric oxide together with a getter substance for a treatment time greater than one hour and at a treatment temperature greater than that temperature required to maintain both the said boric oxide and the said getter substance in a molten state which said removal of impurities results in an improvement in the electrical properties of the said semiconducting material.

2. Method of claim 1 in which said boric oxide, subsequent to said removal of impurities, contains less than 0.01 weight percent of H 0.

3. Method of claim 1 in which said semiconducting material consists essentially of at least one compound selected from the group consisting of gallium phosphide and gallium arsenide.

4. Method of claim 3 in which said getter substance consists essentially of at least one substance selected from the group consisting of gallium, boron-platinum alloy, boron-palladium alloy, aluminum and silicon.

5. Method of claim 4 in which said treatment time is greater than 12 hours and said treatment temperature is between 900 and l, C. 

2. Method of claim 1 in which said boric oxide, subsequent to said removal of impurities, contains less than 0.01 weight percent of H2O.
 3. Method of claim 1 in which said semiconducting material consists essentially of at least one compound selected from the group consisting of gallium phosphide and gallium arsenide.
 4. Method of claim 3 in which said getter substance consists essentially of at least one substance selected from the group consisting of gallium, boron-platinum alloy, boron-palladium alloy, aluminum and silicon.
 5. Method of claim 4 in which said treatment time is greater than 12 hours and said treatment temperature is between 900* and 1,100* C. 